1. Field of the Invention
The present invention relates to a sheet discriminating apparatus for discriminating a sheet type while conveying a sheet, and to an image forming apparatus for forming an image on a sheet which is equipped with the sheet discriminating apparatus.
2. Related Background Art
In a copying machine, a printer, or the like, an electrophotographic image forming apparatus is generally employed. This image forming apparatus transfers a toner image formed on a photosensitive drum or an intermediate transfer member, serving as an image carrier, onto a sheet which is transferred by a pair of rollers as a pair of conveying rotary members. The image forming apparatus heats and pressurizes the sheet by a fixing device to fix the toner image onto the sheet.
It is preferable to fix the toner image onto the sheet with a minimum amount of energy without reducing a degree of fixation of the toner image. However, there is a certain tolerance in heating temperature due to a difference in surface smoothness of the sheet, a difference in thickness of the sheet, and the like. Therefore, the toner image is fixed onto the sheet with low energy efficiency.
In view of the above, there is known an image forming apparatus equipped with a sheet discriminating apparatus for automatically making a discrimination of sheet type (see JP 2001-233500 A). This image forming apparatus forms an image on a sheet after having changed an image forming condition in accordance with a type of the sheet in which a discrimination has been made by the sheet discriminating apparatus.
This sheet discriminating apparatus has an acceleration sensor serving as vibration detecting means, which is provided close to an upstream side of a nip between a photosensitive drum on which a toner image is formed and a transfer roller (or an intermediate transfer roller) for transferring the toner image from the photosensitive drum onto a sheet. The sheet discriminating apparatus detects vibrations of the sheet sent into the nip by the acceleration sensor when the sheet passes after having come into contact with the acceleration sensor, thereby making a discrimination in a type of the sheet.
However, when the sheet is sent into the nip between the photosensitive drum and the transfer roller, it may bend toward or away from the acceleration sensor. Thus, the pressure of contact with the acceleration sensor differs depending on the bending direction of the sheet, so the acceleration sensor may not always be able to detect vibrations of the sheet under a definite condition.